MVVM, MEF, Silverlight and Service Agent

Question

I currently have the following constructors in my viewmodel

    public CartViewModel() : this(new PayPalCompleted()) { }

    public CartViewModel(IPayPalCompleted serviceAgent)
    {
        if (!IsDesignTime)
        {
            _ServiceAgent = serviceAgent;
            WireCommands();
        }
    }

I am trying to modularise my application Prism and MEF. My modules work fine but I'm having trouble with one of my viewmodels.

My problem is that I need to import the EventAggregator at the constructor but I'm having issues about how I do this with a paramaterless constructor as well as an importing constructor

    [ImportingConstructor]
    public CartViewModel([Import] IEventAggregator eventAggregator)
    {
        if (!IsDesignTime)
        {
            _ServiceAgent = new PayPalCompleted();
            TheEventAggregator = eventAggregator;
            WireCommands();

        }
    }

ie I want to do something like this

      public CartViewModel() : this(new PayPalCompleted(),  IEventAggregator  eventAggregator) { }

    [ImportingConstructor]
    public CartViewModel(IPayPalCompleted serviceAgent, IEventAggregator eventAggregator)
    {
             ...stuff
     }

Which isn't correct I know... what is??

Part of the issue, I think, is that when using an importing constructor then the parameters in the constructor are import parameters by default - which would mean that they need a corresponding export for MEF to be able to compose correctly. Which probably means I should export my paypay service? Or should it?

Thanks

Answer 1

The easiest way to deal with this is to expose a property of type IEventAggregator, implement IPartImportsSatisifiedNotification and handle event subscriptions in that method.

Something like this

public class CartViewModel : IPartImportsSatisfiedNotification
{
    private readonly IPayPalCompleted _serviceAgent;

    public CartViewModel(IPayPalCompleted serviceAgent)
    {
        this._serviceAgent = serviceAgent;
        CompositionInitializer.SatisfyImports(this);
    }

    [Import]
    public IEventAggregator EventAggregator { get; set; }

    void IPartImportsSatisfiedNotification.OnImportsSatisifed()
    {
        if (EventAggregator != null)
        {
            // Subscribe to events etc.
        }
    }
}